1. Field of the Invention
The present invention relates to an ultrasonic vibration tool for applying ultrasonic vibration originating from an ultrasonic vibration source evenly across the width of the output end face thereof, and to a fixing device and a heating device employing the same.
2. Description of Related Art
As one of conventional ultrasonic vibration tools capable of applying ultrasonic vibration over a wide width range at one time, there is known an ultrasonic vibration tool 31 as shown in FIG. 9A that is made of a block 32 of substantially rectangular parallelepiped form. The block 32 has its one end face formed as an output end face 33, and has its other end face opposite the output end face 33 formed as an input end face 34, to the substantially central portion of which an ultrasonic vibration source 35 is connected. In this ultrasonic vibration tool 31, a longitudinal standing wave excited by the ultrasonic vibration source 35 is transmitted to the entire width of the output end face 33. The output and input end faces 33 and 34 of the block 32 each have an integral continuous portion serving as a mass portion 36. Between these mass portions 36 are formed slits 37 with a uniform pitch between one another, whereby a plurality of elastic portions 38 are formed.
However, the above stated ultrasonic vibration tool has the following disadvantage. As shown in FIG. 9B, in the ultrasonic vibration tool 31 whose configuration is represented by a dash-and-dot line, the vibrational mode observed when vibration is excited in the central portion of the input end face 34 is represented by a dash-dot-dot line. That is, in the output end face 33, the amplitude of vibration is large in the central portion thereof, but is small in the edge portions thereof. This makes it difficult to obtain a uniform amplitude with high accuracy across the entire width.
To overcome such a problem, for example, an ultrasonic vibration tool as shown in FIG. 10 has been proposed. In this construction, at each edge portion of the input end face is fitted an additional oscillator 39 having a length which is approximately equal to a half of a wavelength, which is called a wave-trapped horn. By exciting the additional oscillator 39 into resonance, the force to excite longitudinal vibration at the edges of the input end face 34 is increased, thereby achieving a uniform amplitude in the output end face 33 (refer to the collected papers presented at the lecture meeting of the Acoustical Society of Japan, pages 737-738, October, 1987, and pages 655-656, March, 1988). However, the additional oscillator 39 tends to cause parasitic oscillation of bending mode and thus fails to achieve a sufficiently uniform amplitude in the output end face.
Such a problem has a significant adverse effect particularly on a fixing device for use in an image forming apparatus which is required to ensure a uniform amplitude distribution with high accuracy.